Update Order Status in Admin and Tax

[daffodil]

Please could anyone point me in the right direction - I'm not looking for someone to fix this, just a pointer in where to look.

I have taxable products set up at 17.5%. All displays fine through checkout, into the admin panel as a new order, but on updating the status of the order to despatched, the tax is charged again.

This is the first ever store I have done that uses the tax module and I am foxed at where to start looking for the error or configuration gaff that I have made (have already searched and am stumped!)

Any hints would be very gratefully received! Thanks